We add the endings -er and -est to words when we want to compare things.
The purple car is old, but the orange car is older and the black car is the oldest.
If the word has a short vowel sound just before the last letter we double the last letter before we add the -er and -est.
big / bigger / biggest
If there are two consonants (words that are not vowels) after the short vowel sound we don't double the last letter.
long / longer / longest
If the vowel sound is long we don't double the last letter.
cheap / cheaper / cheapest
Finally, there are some words that don't obey the rules at all!
good / better / best
bad / worse / worst